North American Fruit Explorers mailing list at ibiblio

[Rodney Eveland <Reveland@Collinscom.net>]

http://instruct1.cit.cornell.edu/courses/hort400/apple2.html

This is a fairly long tutorial: Here is an excerpt from the first page:

Most modern apple production is from clonally propagated fruiting
varieties such as MacIntosh, Red Delicious,
Crispin, etc. grafted (or budded) onto clonal rootstocks selected for
their ability to affect the ultimate size of the tree, disease
resistance, etc. Dwarfing and semi dwarfing rootstock are most commonly
used since they result in smaller, more easily
managed trees with higher overall (per acre) yield than the same
fruiting varieties on their own roots. One disadvantage of
dwarfing rootstocks is that their root system at maturity is smaller,
shallower, and thus less well anchored than vigorous
rootstocks, resulting in a tendency for the tree to lodge (fall over)
especially under a heavy load of apples. An alternative,
though less common, method of achieving size control is double working,
wherein a three part tree is constructed consisting of
a fruiting scion, a size controlling interstock (interstem), and a
vigorous rootstock, which produces a larger and better anchored
root system than a dwarfing rootstock.

Steve Burt's article in the summer Pomona prompted me to search for this
information, although no one has clearly stated just exactly what
diseases the rootstock or interstem can save the scion from. The
tutorial is full of thumbnails which one can click on to see a larger
picture. Makes me wish I was already proficient at grafting..........